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Ross Valley Flood Protection and Watershed Program Flood Zone 9 Advisory Board April 16, 2015 Flow Reduction Study Preliminary Results 1

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This presentation Flow Reduction Study Process Findings of Site Evaluation Proposed Program Alternatives 2

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Overview of Process to Date Review of previous selection of detention basins Developed revised criteria Identified 29 potential alternative flow reduction elements Applied criteria to elements 3

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Reminder: Original Criteria Used To Evaluate Detention Basins Is in close proximity to Corte Madera Creek or a major tributary of Corte Madera Creek. Has the ability to reduce flooding at key flow breakout points along the creeks in Fairfax, San Anselmo, and Ross. Is open lands with few existing structures Has a current use that is compatible with storm water detention. Is publicly owned. 4

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5 Original Detention Basins Selected Phoenix Lake Loma Alta Lefty Gomez Memorial Park Red Hill Park 5

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Ross Valley Flow Reduction Criteria -15 Criteria 5 Categories (Details in Table 1) A. Reduce Flooding Potential B. Protect Environment C. Optimize Costs and Benefits D. Gain Public Acceptance E. Optimize Schedule 6

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Proposed Category Weighting A. Reduce Flooding Potential Critical Success of Project (5) B. Protect Environment Very Important to Success of Project (4) C. Optimize Costs and Benefits Very Important to Consider (3) D. Gain Public Acceptance Very Important to Consider (3) E. Optimize Schedule - Important to Consider (2) 7

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Refreshed List of Flood Control Alternatives (Table 2) Detention Basins 5 Original Recommended Basins (Phoenix, Lefty Gomez, Loma Alta, Memorial Park, Red Hill Park) 14 Additional Detention Basin sites See Map Other Elements Creek Widening Bypass Channels Setback Levees Utilize Surface Streets as Shallow Channels Removal of Flood Prone Properties Flood Proofing Implement LID Measures 8

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Fatal Flaw Analysis 3 Sites Eliminated Fatal Flaw Criteria A3. Avoid Flooding Downstream Potential to Increase Flow Downstream B3. Increase Ability to Obtain Permits (In)Ability to Gain Permit Approvals D1. Maximize Public Acceptance Public Safety potential to trap people. Eliminated Sites U. Detention Facilities on Cascade Creek K. Underground Parking Detention. W2. Marin Town Country Club (Cistern) W3. Marin Town Country Club (Underground parking) 11

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Site Evaluation Summary Top 10 Sites (Details in Table 2) 1.Phoenix Lake (710) 2.Memorial Park (620) 3.Deer Park (611) 4.Nursery near Baywood Canyon Dr. (611) 5.Bothin Park Youth Center (581) 6.San Domenico School/Upstream (580) 7.Lefty Gomez (573) 8.Marin Town and Country Club Detention Pond (572) 9.Red Hill Park (569) 10.Hidden Valley (568) 12

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Developing Program Alternatives Group highest ranked elements in various combinations that: 1.Are anticipated to meet flow targets throughout the basin (Shown on Figure 2) 2.Distribute flow detention upstream of key flooding locations throughout the watershed 3.Include a range of the top ranked detention basin sites. 13

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Three Program Alternatives 1 - The Highest Ranking Elements (based on individual element scores) 2 The San Anselmo Option (compare flood protection for Sleepy Hollow Creek vs. Sorich Creek, Alternative 1 vs. Alternative 2) 3 Most Environmentally Friendly (no detention basins in protected spaces or reserves) 15

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Next Steps Continue to invite public comments Hydraulic modeling of the three alternatives Reconfiguring of the alternatives to optimize performance Prepare conceptual plans and cost estimates for new elements Comparison of alternatives to existing Program 17

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Schedule DeliverableTWG Presentation Date AB Presentation Date Selection criteria, weighting factors, and list of Flow Reduction Elements being considered. February 26, 2015 March 17, 2015 Results of applying fatal flaw criteria to Flow Reduction Elements; group surviving elements into Alternatives March 26, 2015 TODAY- April 16, 2015 Analyze and apply criteria to Alternatives, and rank them MayNo Presentation Report with recommendations, conceptual plans, and cost estimates. No Presentation June 18

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Questions? 19